Gray iron is a desirable casting material because its excellent castability and low cost makes it versatile for the manufacture of products such as crank cases and cylinder heads. Such manufacturing components require high strength, soundness, good machinability, dimension stability and uniform properties. To obtain these qualities, it is important to achieve a uniform metallurgical structure throughout all sections of the casting and particularly a uniform pearlite structure. Alloys are commonly added to gray cast iron materials in the casting process in an effort to achieve these desirable properties, and the effect of alloying on gray iron has been extensively studied, as indicated, for example, by “A Modern Approach To Alloying Gray Iron,” Janowak & Gundiach, AFS Transactions, Vol. 90, 1982, and “Effect of Manganese and Sulfur on Mechanical Properties and Structure of Flake Graphite Cast Irons,” Fuller, AFS Transactions, Vol. 94, 1986.
Notwithstanding this prior work, in order to meet strength requirements, gray iron castings for crank cases and cylinder heads were manufactured by alloying molten gray iron with chromium, but this caused hard spots in the casting due to chilling and iron carbide formation, which resulted in machining difficulties, damaged castings, and poor cutting-tool life and performance. In an effort to partially reduce the chilling tendency, silicon levels in the gray iron base and additions of a silicon-based inoculant were increased in the molten gray iron, which significantly increased the cost of manufacture. Furthermore, the additional silicon increased the need for a chromium strengthening alloy, which further increased costs and the tendency to form iron carbide hard spots and chills. In addition, as a result of the high level of alloying, solidification stresses in the resulting crank cases and cylinder heads were high, requiring stress relief heat treatment to minimize distortion and cracking of the castings during processing. Casting stresses were further increased by the need for the relatively low temperature shake-out, that is extraction from the mold, at temperatures of 900°–1200° F.
Thus, a need remained for a method of economical manufacturing for crank cases and cylinder heads using gray cast iron with a minimal formation of iron carbide hard spots and chills upon solidification, and a minimal need for stress relief heat treatment of the finishing casting.